Cerberus and Ahriman

Prior to charging on, you'll want to strengthen your party's defenses by equipping magical damage reduction gear. The specific element doesn't matter, as you'll be dealing with every flavor. As for your attackers, they may be whomever you feel is most powerful, regardless of how they engage foes—the next boss, unlike King Behemoth and the Iron Giant, does not counterattack.

Straight west of the Save Point, you will find another crimson-colored crystal, which hides the hellbeast Cerberus. Per normal, you should Slow them, use Haste on your damage dealers and support classes, and supply four layers of Protect and Shell for everyone. Once you have finished buffing them—and if you have a healthy stock of Hi-Potions—you may want to cast Blink and Reflect, too, as Cerberus will practically only sling spells.

With the aforementioned defenses, you may destroy Cerberus without taking any damage. Use your fiercest fighters' high-level spells (e.g., Flare) and Band Attacks, and the three-headed mutt will go down.

From Cerberus' dying place, travel directly east to find Ahriman, who should be the nastiest representative of its species you'll face, but its list of spells is pathetically limited. Their fight will be opened with Doom, which is pleasant enough. After using Slow on the winged menace, use the very same spell on a few of your teammates. This will ensure that they outlive some of your companions, leaving someone behind to start Raising party members.

Take note, however, of Ahriman's casting pattern. If the Doom status effect is present on anyone party members—no matter how few—they will not try casting again until they die. You may also try casting Hold upon someone suffering Doom whom you shan't need, as this should free up everyone else to fight without fear.

As for how you'll hurt it, it's the same as before: high-level spells like Flare, and heavy damage Band Attacks.